The Japanese unexamined Patent Publication No. 173116/84 and other literatures already describe processes for separation and purification of a product gas by the Pressure Swing Adsorption method, starting with gas mixtures containing carbon dioxide, methane or other gas components which can be adsorbed relatively easily. The separation and purification processes of carbon dioxide, methane or the like by the PSA method, generally use more than one adsorption tower in which, individually, the pressurization/adsorption step, the washing step, and the desorption step are repeated, and which altogether are operated in combination. After adsorption is completed in a tower, a part of the gas desorbed in another adsorption tower is sent to the adsorption tower to flush the interior, and then, the gas is desorbed at a reduced pressure, a part of which is continuously recovered as product, while the balance is used for flushing of another adsorption tower.
In the conventional process, a large-size unit had to be used to obtain the product gas at high purities, for example, 99% or more and preferably 99.5% or more, because the desired product gas was easily contaminated with other gas components. In order to recover the desired gas component at high purity, it was necessary to flush the interior of the adsorption tower with the same desired gas of relatively high purity for a considerable time, thus flushing the voids with such high purity gas as well as purifying the gas adsorbed on the adsorbent through consequential rise of partial pressure of the desired gas component. This operation resulted in a reduced separation/purification capacity per unit adsorbent. Therefore, the size of the unit had to be increased in order to maintain the capacity, and this fact has posed a problem in the industrial application of the process.